The invention disclosed herein are for improving the performance of digital fluorography systems.
This specification corresponds to the specification in copending applications: Ser. No. 400,550, filed July 21, 1982; and, Ser. No. 400,552, filed July 21, 1982; which are assigned to the assignee of this application.
Digital fluorography procedures are used for visualizing blood vessels in the body. X-ray images are acquired by projecting an X-ray beam through a region of interest in the body and using an image intensifier to convert the X-ray image to an optical image. The optical image is viewed by a television camera which converts each image frame to corresponding analog video signals. The analog signals are converted to digital signals corresponding, respectively, in value to the intensities of the picture elements (pixels) that compose the image.
In the procedure for enabling visualizing or display of a blood vessel at least one image, called the mask image, containing the blood vessel and the tissue or bone in its background is obtained before a previously intravenously or arterially injected X-ray contrast medium reaches the blood vessel in the region of interest. This mask image is digitized and stored in a full-frame digital memory. After a short delay following the mask or pre-contrast image, the X-ray contrast medium such as an iodinated compound, begins to flow through the blood vessel. When flow begins a series of additional images, called post-contrast images, are obtained usually at about one second intervals whereupon contrast medium concentration maximizes and finally disappears from the blood vessels. These live post-contrast images are subtracted successively from the mask image and the resulting difference images are stored, usually in an analog video disk recorder or a digital disk memory system. The subtraction process that produces the sequence of difference images is expected to cancel out bone and soft tissue and anything that remains constant between successive images while at the same time letting the contrast medium remain for defining the walls of the blood vessel. If there has been no patient movement between the time the mask image and the post-contrast images are obtained, the difference image that corresponds to maximum contrast medium concentration will ordinarily exhibit the best contrast resolution when the image is displayed on the screen of a television monitor. There are, however, occasions when the maximum contrast difference image or images show artifacts which may be due to body movement during the time between mask image acquisition and the post-contrast images.
Artifacts may result from other causes, too. If no difference image appears to have good enough contrast resolution, a reprocessing procedure is used to obtain such a new difference image in a manner that avoids repeating the X-ray exposures. Reprocessing involves selecting one of the post-contrast images for use as a mask image in place of the original mask. The stored difference images resulting from the first sequence of subtractions are displayed and one is selected near the beginning of the post-contrast interval where little contrast medium is present in the blood within the vessel. Other difference images in the sequence of post-contrast difference images are tested by subtracting them from the newly selected mask image. There is a high probability that at least one of the resulting subtracted images will be without artifacts. In other words, it is likely that a new mask image and another post-contrast image will be found where motion artifacts will be in registry so they will subtract out and let an image of the blood vessel that has satisfactory contrast resolution remain.
To get good contrast resolution in a difference image, it is imperative for the digital pixels in image frames that are to be subtracted to be registered with each other. It is also important that any artifacts which may appear in the video information are the same for pre-contrast, post-contrast and difference images that are to be subtracted. A further requirement is that the vertical and horizontal scanning rates of the television camera acquiring the images remain the same. The process of storing the images on analog video disk and reading out the images from the disk must also be timed in an accurate and reproducible manner. None of these objectives have been satisfactorily achieved until the inventions disclosed herein were made.
Conventional television practices are not satisfactory for digital fluorography systems. They are incapable of obtaining registration of artifacts between successive images so the artifacts cannot be cancelled by subtracting images. The artifacts most difficult to deal with are those that result from stray electrostatic and magnetic fields and other interference that orginates in the ac power lines. Stray fields influence the electron beam that scans or reads out the target of the television camera tube. A weak stray field can deflect the scanning beam by a significant amount where the beam velocity is near zero as it is near the target. The amplitude of the analog video signals can be affected adversely by ac line hum, power supply ripple and electrostatic and magnetic interference. The waveforms of the signals used to sweep the scanning beam may develope glitches due to interference or noise pickup in the scanning coils or electrodes. All of these factors cause greater image quality problems in a digital fluorography system because after the live digitized images are subtracted from the mask, the result must be enhanced or subjected to digital gain. This greatly emphasizes any small artifacts that would not even be seen in cases where live images are displayed directly from a television (TV) camera.
The various interference artifacts are manifested in prior art fluorography systems as a series of regions or hum bars in the displayed image which are not exactly registered with the mask image because the TV camera sync frequencies would not be the same as the power line frequency and the misregistration bars would then roll through the image at a frequency equal to the difference between TV camera vertical frequency and power line frequency. Line-locking methods used thus far suffer either from image size changes or inadequate accuracy.